This invention relates to the preparation of thermally stable, substantially polycrystalline silicon carbide ceramic fibers using borosilazane resins. The method described herein provides a simple yet effective method of preparing desirable fiber at a relatively low cost.
Silicon carbide ceramic fibers are known in the art for their mechanical strength at high temperatures. Because of this property, they have found utility in numerous areas such as reinforcement for plastic, ceramic or metal matrices to produce high performance composite materials or the formation of fibrous products such as high temperature insulation, belting, gaskets and curtains.
Several methods have been developed to manufacture such fibers. For instance, it is known that organosilicon polymers may be spun into fibers, infusibilized (cured) and pyrolyzed at elevated temperatures to form a ceramified fiber. Unfortunately, this method suffers from the disadvantage that substantial amounts of oxygen and/or nitrogen are often incorporated into the fibers either directly through the polymer or indirectly by incorporation during spinning, infusibilization or ceramification. When these fibers are heated to temperatures above 1400.degree. C., the oxygen and nitrogen is lost causing weight loss, porosity and decreased tensile strength.
Recently, polycarbosilane preceramic polymers having a Si-C skeletal structure have been used to minimize the incorporation of nitrogen and oxygen. Yajima et al. in U.S. Pat. Nos. 4,052,430 and 4,100,233, for example, teach a method of producing silicon carbide fibers by spinning, infusibilizing and pyrolyzing various polycarbosilanes. Nippon Carbon Co., moreover, utilizes this technology to produce a SiC ceramic fiber sold under the trade name NICALON.TM.. These fibers too, however, are known to contain about 9-15% oxygen and, thus, degrade at temperatures as low as 1200.degree. C. (see Mah et al., J. Mat. Sci. 19, 1191-1201 (1984)
Borosilazanes are also known in the art. For instance, Japanese Kokai Patent No. Hei 2-84437 describes the formation of borosilazanes by reacting a silazane with a boron compound. The reference describes the resultant materials as useful in the formation of ceramics. In "International Symposium on Organosilicon Chemistry Directed Towards Material Science", Abstracts, P. 95-96, Sendai, Japan, 25-29 Mar. (1990), the same inventors describe the use of these materials in the formation of fibers. However, since the fibers contained insufficient carbon and were only pyrolyzed up to 1700.degree. C., the resultant product is described as an amorphous silicon boron nitride fiber. The fibers claimed in the present application, on the other hand, are predominantly crystalline, silicon carbide.
European Patent No. 364,323 describes a method of forming polymers based on boron and nitrogen comprising reacting a silazane with a trihalogen borane. The resultant materials are taught therein as being useful in the formation of boron nitride ceramics, including fibers.
Takamizawa et al. in U.S. Pat. No. 4,604,367 teach the preparation of an organoborosilicon polymer by mixing an organopolysilane with an organoborazine compound, spinning fibers and then ceramifying the fibers by heating to temperatures in the range of 900.degree.-1800.degree. C. The organoborosilicon polymer therein, however, is described as having a skeletal structure comprising Si, C, N and B compared to the Si, B and N chains of the present invention. Moreover, this reference teaches that the tensile strength of the fibers drops off dramatically when heated above 1500.degree. C. (note the graph on the cover of the reference)
U.S. Pat. No. 4,910,173 granted to Niebylski describes the formation of organoborosilazane polymers by the reaction of a polysilazane with a boroxine. The reference states that the resultant materials are useful in the formation of ceramic fibers but fails to teach the methodology.
Seyferth et al., J. Am. Ceram. Soc. 73, 2131-2133 (1990) likewise teach the formation of borosilazane polymers by the reaction of silazanes with boranes. The reference teaches that the resultant materials are useful in the production of borosilicon nitride ceramic fibers.
The present inventors have now unexpectedly found that thermally stable, substantially polycrystalline SiC fibers can be formed by firing borosilazane fibers having greater than about 0.2% boron and greater than about 0.1% free carbon incorporated therein to a temperature greater than about 1700.degree. C.